The invention relates to a process for producing leached fiber bundles (LFBs), and to an improved LFB.
LFBs are used in particular as image light guides for the transmission of optical information, for example in endoscopes. LFBs generally include a multiplicity of optical fibers which are arranged in defined fashion. The diameter of the optical fibers is typically 8 to 120 μm. The optical fibers themselves often comprise a light-conducting core and a sheath, for example a glass or plastic, with a lower refractive index.
Processes for producing LFBs are known, for example from WO 02/40416 A1 and U.S. Pat. No. 4,389,089. To obtain an ordered fiber arrangement, LFBs are produced by drawing out correspondingly arranged fiber bundle preforms, for example glass rods or tubes, which additionally include at least a few spacers made from etching glass, i.e. a glass which can be partially dissolved by treatment with acids, bases or deionized water. The spacer preforms are generally in the form of tubes or rods. The distance between and arrangement of the optical fibers with respect to one another can be defined by a suitable arrangement of the etchable spacers.
The fiber bundle preforms obtained in this way are then drawn out under the action of heat using known processes until the desired fiber or fiber bundle cross section is reached. In the process, the spacers are fused to the optical fibers and fill the space between the individual optical fibers.
To produce an image light guide, the ends of the fused fiber bundles are provided with a protective layer which is resistant to acids and/or lyes, and the spacers located in the unprotected regions are removed in hot acid or lye baths or baths in deionized water. In this way, the optical fibers are uncovered or separated from the spacers again and the fiber bundle becomes flexible at these locations. The ends remain connected to one another and therefore rigid and fixed. The end faces are also usually polished in order to increase the optical quality.
In many applications, for the optical quality it is also important for the light outlet points, i.e. the end faces of the optical waveguides to adopt an accurately defined position in the matrix of the end faces of optical fibers and spacers.
The production process described has the drawback that, on account of the optical fibers and the etching glass fusing together in the end region of the fiber bundle, the fibers are shifted out of their original geometric position, and consequently an accurate position of the light outlet points at the end of the optical fiber bundle is lost. For applications which require a high image or light transmission quality, for example connecting units for optical data transmission (optical interconnects), therefore, it is not possible to use this process for producing optical fiber bundles.
A general problem with optical fiber bundles is the fact that their ability to withstand mechanical loads is often low. If fiber bundles are bent, the individual fibers rub against one another, which leads to an increased mechanical stress which can ultimately cause fibers to break. If dirt particles are present between the individual fibers, the stresses are increased further. Each broken optical fiber leads to the associated light point failing, so that if a corresponding number of fibers have broken, the entire fiber bundle has to be exchanged. To extend the service life of the optical fiber bundle, WO 02/40416 A1 proposes introducing a special powder between the individual fibers of the bundle and placing a protective mesh around the flexible region of the optical fiber bundle. However, even these measures are for certain applications not sufficient to effect sufficient durability of the fiber bundles.